Barry
Unfortunately, with no other facts it is impossible to nail down the cause for the crashes. I also run MSM & Mach3 ver.066 and the smooth stepper but on a Pro.
The error message your getting is so generic as to be useless. I get the same error message every few days but as luck would have it, only on shut down of Mach.
Otherwise it is performing flawlessly. You might try the Mach forum on CNCZone or their Yahoo forum.
Sorry I couldn't help.
John